1. Field of the Invention
This invention relates to an image information transmitting system and more particularly to a system for continuously transmitting a temporally correlated group of image planes.
2. Description of the Related Art
In transmitting information such as image information, it is always the theme of efforts to reproduce the original information with a higher degree of fidelity with a smaller amount of transmitting information. Hence, varied kinds of transmission methods have been proposed for this purpose.
These methods include adaptive type variable density sampling methods of appropriately changing sampling density, that is, varying the density of information being transmitted. An example of this method has been disclosed and known by the name of a time axis transforming band compressing method (hereinafter referred to as TAT method) The TAT method is briefly described below:
FIG. 1 of the accompanying drawings shows the fundamental concept of the TAT method. An original signal is divided as indicated by broken lines into blocks by a predetermined period of time. The information contained in the original signal within each divided block is checked to discriminate its degree of density. When any of the blocks is thus found to be dense, data obtained by sampling the original signal of the block is completely transmitted as transmission data. For a block determined to be sparse, only a portion of data is transmitted while the rest is regarded as skipped data and is not transmitted.
The arrangement according to this concept decreases the amount of data to be transmitted per unit time and thus permits the transmission signal to be band compressed. The data thus transmitted is used by the receiving side for forming data corresponding to the skipped data. In other words, some interpolation data which is in proximity to the skipped data is obtained by computation by using the transmitted data. Since the interpolation data corresponds to a sparse part of the information signal, it is in close proximity to the skipped data. Compared with a case where the whole data is transmitted, the interpolating arrangement gives a reproduced signal with a fairly high degree of fidelity to the original signal while the transmission band can be reduced to a great degree by the arrangement. In other words, the amount of information to be transmitted is reduced by the arrangement.
Meanwhile, the elaborateness or fineness of the original signal within each of the divided blocks are examined in making a discrimination between transmitting the whole sampling data and transmitting just a portion thereof. Information on the result of this discrimination is also transmitted along with the transmission data as transmission mode information.
In the case of image information, transmission according to the above-stated concept is performed in the following manner: The image information has a two-dimensional spread and has a correlativity between horizontal and vertical directions. Therefore, transmission of image information can be more effectively accomplished by arranging the intervals of sampling to be variable not only in the horizontal direction but also in the vertical direction. This idea will be called the two-dimensional TAT method. The following is the brief description of the two-dimensional TAT method:
FIG. 2 is a data transmission pattern of the two-dimensional TAT method. In this method, one picture plane is divided into a plurality of picture element blocks. Each of the divided blocks consists of an m.times.n number of picture elements. The transmission data density of one picture element block is arranged to be variable from another and independently of another. In the case of FIG. 2, each picture element block consists of 4.times.4 picture elements and is arranged to be transmissible in two different transmission modes. In FIG. 2, each mark " .circle." represents a picture element to be transmitted and another mark "X" a picture element to be skipped. A reference symbol E denotes a transmission pattern in which data of all the picture elements is transmitted; and another symbol C a pattern in which only a portion of data of all the picture elements within one block is transmitted. Hereinafter, the mode of transmission in the former pattern will be called the E mode and transmission in the latter the C mode respectively. As apparent from the illustration, data is transmitted in the C mode with 1/4 of the information transmitting density of the E mode. In the case of the C mode, the original image plane is reproduced by forming interpolating picture element data for each of the skipped picture elements on the basis of the transmitted data representing a picture element located near to the skipped one within the same picture element block. A system for carrying out the two-dimensional TAT method is arranged as described below with reference to FIG. 3:
FIG. 3 is a block diagram showing by way of example a analog transmission system. An incoming video signal is sampled for all the picture elements thereof by an analog-to-digital (hereinafter referred to A/D) converter 1. By this, data for all the picture elements is generated. This all-picture-element data is supplied to a skipping circuit 2. The circuit 2 performs a skipping operation in a manner corresponding to the C mode pattern shown in FIG. 2. The circuit 2 thus produces C mode picture element data. The C mode picture element data is supplied to an interpolation circuit 3, which performs computing operation to obtain interpolation picture element data corresponding to the skipped picture elements. The interpolation picture element data is supplied to a mode discrimination circuit 4 together with the all-picture-element data produced from the A/D converter 1. Then, each picture element block is determined whether it is to be transmitted in the C mode or in the E mode. At the mode discrimination circuit 4, computation is performed for each of the picture element blocks to obtain a difference between the picture element data produced from the A/D converter 1 and the interpolation picture element data. The sum of the difference (hereinafter referred to as a block distortion) is computed for every picture element block and then a total difference thus obtained for one field portion of the signal is stored in a memory.
Before arrival of the data of a next field, the distribution of block distortions of all the picture element blocks is thus obtained. In this instance, the ratio of the number of picture element blocks to be transmitted in the C mode to that of the picture element blocks to be transmitted in the E mode must be arranged to be unvarying. For example, assuming that 2/3 of all the picture element blocks are to be transmitted in the C mode and 1/3 of these blocks to be transmitted in the E mode, a total number of transmission data (or the rate of compression) becomes (2/3.times.1/4+1/3.times.1=)1/2. Therefore, in accordance with the distribution of the block distortion covering all the picture element blocks, a threshold value of distortion is predetermined for determining a boundary between the C mode and the E mode.
Following this, at the time of arrival of the incoming video signal for the next field, the stored block distortion values are read out one after another and compared with the threshold value to determine thereby the transmission mode to be selected. In case that the read out distortion value coincides with the threshold value, the transmission mode is determined in such a manner that the number of the picture element blocks to be transmitted in the C mode and that of the blocks to be transmitted in the E mode are in the predetermined ratio.
A mode discrimination signal which is thus obtained in the above-stated manner is supplied to a switch 7. Then, the picture element data is selectively read out from a buffer 5 which is provided for the picture element data of the E mode and a buffer 6 which is for the picture element data of the C mode. The output of the switch 7 is supplied as the transmission data to a digital-to-analog (D/A) converter 8 to be converted back into an analog video signal. This signal is then produced to a transmission line. Further, the mode discrimination signal is also produced to the transmission line via a buffer 9 as a mode signal.
FIG. 4 shows in outline the arrangement of the receiving side of the two-dimensional TAT transmission system. The video signal which has been processed in the manner as described in the foregoing and supplied via the transmission line is received at an A/D converter 10 to be converted back into a digital signal. The output of the A/D converter 10 is supplied to a C mode interpolation circuit 11. The circuit 11 performs a computing operation to obtain interpolation data corresponding to the skipped picture element data in the C mode.
Meanwhile, the transmitted mode discrimination signal or mode information signal controls a switch 12. The connecting position of the switch 12 is shifted to its one side E when the signal indicates the E mode and to the other side C thereof when the signal indicates the C mode. Through this switch 12, the whole picture element data including the E mode picture element data, the C mode picture element data and the interpolation picture element data is stored gradually at a frame memory 13. The stored data is read out from the frame memory 13 in a sequence, for example, according to a television signal. The read out data is produced via a D/A converter 14.
As described above, the image information can be effectively transmitted by the transmission system operating according to the two-dimensional TAT method. However, when a television signal which is obtained in the manner described above is displayed, deterioration becomes conspicuous in resolution in a still picture region although the resolution is acceptable in a motional picture region. Meanwhile, in the still region on the image plane, there is a high correlativity in the time axial direction. A method of utilizing this correlativity in the time axial direction has recently advanced.
In the transmission system embodying the two dimensional TAT method mentioned in the foregoing, there is provided no particular arrangement to make a distinction between the still region or part and a motional part of the image plane even in continuously transmitting a group of image planes having a temporal correlation among them. Therefore, in the event of a still part having an extremely high degree of correlativity on the time base or time axis, similar image information is repeatedly transmitted many times and thus results in a very poor transmission efficiency. Further, on some occasions, a still picture region is not adequately reproduced. In case that the whole image plane is a still picture in particular, the deterioration of resolution becomes salient in the C mode transmission part.
The still picture region is not always completely motionless but there are some occasions on which some motion is included in a still picture region. Hence, it is extremely difficult to determine the information on each picture element block as to whether it is to be transmitted or not to be transmitted.
Let us assume a case wherein comparison is made between two temporally continuing image planes to obtain a difference between their picture elements and further to obtain data (temporal distortion) for temporal correlativity by summing up the difference between the whole blocks. It is then conceivable to determine transmission or no transmission for each picture element block on the basis of the data obtained in this manner. In that case, however, there arises the following inconvenience:
For example, there is a case where a picture of two-dimensional frequency such as a background scenery slowly moves while, in another case, an extremely small object moves over a picture plane. In the former case, the above-stated temporal distortion becomes conspicuous. The system is arranged to transmit no information about the former image or picture plane while information about the latter image plane is transmitted as the small object is not readily observable on the image plane and tends to bring about some discrepancy between it and the above-stated temporally distorted data.
To enhance transmission efficiency, it is conceivable to make variable the sampling period within the frame of continuing image planes and in the time axial direction by utilizing both the correlativity within each frame and the correlativity between frames. In that case, however, there arises the following problem:
With variable density sampling performed in the time axial direction, the amount of image information per frame generally comes to vary. In cases where image information for which the transmission time per frame is predetermined and fixed as in the case of a television signal is to be transmitted, arrangement to have a fixed interval at which each data is transmitted hinders adjustment of transmission time and thus makes system arrangement difficult. Further, the arrangement to make the data transmission interval variable results in the complex hard-ware arrangement of the transmission system.
To solve this problem, it is conceivable that, for the still picture region, a process utilizing the temporal correlativity, such as composing a reproduced image plane by using the data of an image plane transmitted immediately before, is arranged to be performed on the receiving side. However, in such a system, the arrangement to determine each block to be transmitted in the E mode or in the C mode in the same manner as in the case of the above-stated two-dimensional TAT method comes to present the following problem:
Considering now two picture element blocks within an image plane, let us assume that one of the blocks represents a highly elaborate portion of the image plane but little varies from a preceding image plane while the other is not so elaborate and varies from the preceding image plane. In this instance, the former is transmitted in the E mode and the latter in the C mode in accordance with the two-dimensional TAT method described. Then, with the process utilizing the temporal correlativity performed on the receiving side, the former picture element block can be reproduced in a image which little varies from an image obtainable from transmission in the E mode. Whereas, with the latter picture element block transmitted in the C mode, it hardly can be reproduced with a high degree of resolution even through a spatial interpolating process or a process utilizing the temporal correlativity. In other words, in order to obtain a reproduced image plane with high resolution, the transmission in the E mode should be performed for the latter picture element block rather than for the former. Therefore, in this instance, the determination of the transmission mode in accordance with the two-dimensional TAT method becomes irrational.